A、On short trips. B、On long trips. C、When flying over cities. D、When flying at high altitudes. A

Although I think the United States generally has an excellent system of transportation, I do not think that it does a good job of transporting people between cities that are only a few hundred miles apart. A person commuting between Detroit and Chicago, or between San Francisco and Los Angeles, so-called "strip-cities", may spend only a relatively short time in the air while spending several hours getting to and from the airport. This situation makes flying almost as time-consuming as driving. Moreover, airplanes use a lot of their fuel just getting into the air. They are simply not fuel-efficient on short trips. High-speed trains may be an answer. One fairly new proposal for such a train is for something called "mag-lev", meaning a magnetically levitated train. Mag-levs will not actually ride on the track, but will fly above the track that is magnetically activated. This will save wear and tear on the track. These trains will be able to go faster than 150 miles per hours. At that speed, conventional trains have trouble staying on the track. As you can see, mag-levs offer exciting possibilities for the future.